Spark plug

Abstract

A spark plug having a metal shell, center electrode and a ground electrode. The ground electrode includes an electrode base portion extending from a front end portion of the metal shell toward the front, a bent portion connected at one end thereof to a front end of the electrode base portion and an electrode distal end portion extending from the other end of the bent portion and forming a spark discharge gap with a center electrode. The ground electrode has a base material and a coating layer applied to the base material. The coating layer is formed on at least a front end face and an outer circumferential surface other than a center-electrode-side surface of the electrode distal end portion of the ground electrode. The base material of the ground electrode is exposed at least at a part of the electrode base portion.

Having described the invention, the following is claimed: 
     
       1. A spark plug, comprising:
 a cylindrical insulator having an axial hole formed therethrough in an axis direction of the spark plug; 
 a center electrode inserted in a front side of the axial hole; 
 a cylindrical metal shell arranged on an outer circumference of the insulator; 
 a ground electrode comprised of a base material, said ground electrode having a first end joined to a front end portion of the metal shell and a free end forming a gap with the center electrode, said ground electrode having an electrode base portion, a curved bent portion and an electrode distal end portion, the electrode base portion being joined to the front end portion of the metal shell and extending from the front end portion of the metal shell toward a front end of the spark plug in the axis direction; the curved bent portion being connected at one end thereof to the electrode base portion and connected at another end to the electrode distal end portion, said electrode distal end portion extending from the bent portion in a direction different from the direction of the electrode base portion, said distal end portion having a distal end face at the free end of said ground electrode, and forming the gap with the center electrode, 
 a coating layer applied to the base material of said ground electrode, said coating layer made of a material having higher oxidation resistance than that of the base material, said coating layer applied to the face end of the ground electrode so as to cover the distal end face and an outer circumferential surface of said ground electrode except a surface of the electrode distal end portion facing the center electrode; and 
 wherein at least a part of the base material of the electrode base portion of the ground electrode is not covered by said coating layer. 
 
     
     
       2. The spark plug according to  claim 1 , wherein the base material of the ground electrode is exposed at the entire outer surface of the electrode base portion. 
     
     
       3. The spark plug according to  claim 1 , wherein the coating layer is formed only on the electrode distal end portion; and wherein the base material of the ground electrode is exposed at the bent portion. 
     
     
       4. The spark plug according to  claim 1 , wherein the base material of the ground electrode is a metal material containing 90 mass % or more of nickel (Ni). 
     
     
       5. The spark plug according to  claim 1 , wherein the coating layer has a thickness of 5 to 60 μm. 
     
     
       6. The spark plug according to  claim 1 , wherein the coating layer is formed only on the electrode distal end portion, or is formed on the electrode distal end portion and the bent portion in such a manner that a minimum thickness of the coating layer on the electrode distal end portion is larger than a minimum thickness of the coating layer on the bent portion. 
     
     
       7. The spark plug according to  claim 1 , wherein a minimum thickness of the coating layer on the distal end face of the electrode distal end portion is larger than a minimum thickness of the coating layer on the outer circumferential surface other than the center-electrode-side surface of the electrode distal end portion. 
     
     
       8. The spark plug according to  claim 1 , wherein the coating layer is formed only on the distal end face and the outer circumferential surface other than the center-electrode-side surface of the electrode distal end portion, or is formed on the entire outer surface of the electrode distal end portion in such a manner that a minimum thickness of the coating layer on the center-electrode-side surface of the electrode distal end portion is smaller than a minimum thickness of the coating layer on the distal end face and the outer circumferential surface other than the center-electrode-side surface of the electrode distal end portion. 
     
     
       9. The spark plug according to  claim 1 , wherein the material of the coating layer is a material containing Ni, cobalt (Co) and chromium (Cr). 
     
     
       10. The spark plug according to  claim 9 , wherein the material of the coating layer further contains yttrium (Y) and aluminum (Al). 
     
     
       11. The spark plug according to  claim 1 , wherein the coating layer is formed by high velocity oxygen fuel (HVOF) spraying, high velocity air fuel (HVAF) spraying, plasma spraying, cold spraying or aerosol deposition.